We used data from the 1998–2009 waves of the National Health Interview Survey to investigate cohort differences in low birthweight among US‐born children of mothers arriving in the United States between 1955 and 2009, cohort‐adjusted patterns in low birthweight by maternal duration of residence in the United States, and cohort‐adjusted patterns in low birthweight by maternal duration of US residence stratified by age at arrival and region of origin. We found a consistent deterioration in infant health with successive immigrant cohorts and heterogeneous effects of cohort‐adjusted duration in the United States by age at arrival and region. Most notably, we found evidence that maternal health (as proxied by low birthweight) deteriorates with duration in the United States only for immigrant mothers who came to the United States as children. For mothers who arrived as adults, we found no evidence of deterioration. The findings underscore the importance of considering age at arrival and place of origin when studying post‐migration health trajectories and provide indirect evidence that early life exposures are a key to understanding why the United States lags other developed nations in health.